What does this mean at home? An urgent bulletin has gone out to police forces across the country to be on alert more possible Isis attacks. ABC's senior justice correspondent Pierre Thomas joining us from Washington. Pierre has a legitimate threat been made. Reporter: No specific threat has been identified but they're concerned that they have sympathizers here in the U.S., so-called lone wolves. More than 100 Americans have gone to join forces with radicals in Iraq and Syria. My sources believe that this proves there's clearly a pool of people here in the U.S. Who identify with these foreign fighters and that could be dangerous. You mentioned those lone wolves and what is the biggest concern about that group? Reporter: These are people the government may not have identified who could just be watching the events unfold from Iraq and not be officially tied to Isis at all. They could come out of nowhere. Paula. All right, Pierre Thomas, thank you for that report this

This transcript has been automatically generated and may not be 100% accurate.

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